This invention relates to high solids concentration slurries showing shear-thinning visco-plastic Theological properties and, more specifically, to a process and apparatus which allows for improved transfer of such thick, high solids content slurries or mud from a reservoir to a conduit or any other transportation device.
A large number of industrial processes generate solids residues which normally have to be washed and transported to disposal sites or to other recycling processes. The most common method used in industry to transport these solids residues is to slurry them in a carrying medium, water being the most commonly used medium, and to pump them to the desired destination. If the slurry is sufficiently concentrated, other means of transfer, such as belt conveyors, can also be used. It is important and most desirable, from an economical point of view, to operate with slurries of the highest solids concentration possible. This keeps the total amount of material to be handled, and eventually to be disposed of, to a minimum. When washing is necessary, a high solids content allows a maximum washing efficiency in the minimum number of washing stages. Finally, maximizing the solids content of a slurry has a favorable impact on the environment by reducing the total amount of material at the disposal site and reduces the risk of spillage and leakage of liquid effluents.
Achieving a high solids concentration from a dilute slurry can be done in numerous ways, the most common ones being by means of filtration, hydrocyclone, centrifuge, flotation, magnetic separation or gravity settler also called decanter or thickener. Each of these methods is capable of producing, to various degrees, a slurry of sufficiently high concentration that the thickened slurry or resulting mud will behave as a paste or a compacted cake. If the solids concentration achieved is very high and a relatively dry cake is formed, conventional dry transportation systems, such as belt or screw conveyors can be used. This is the case, for example, when a high efficiency vacuum filter or a pressure filter, such as a plate and frame type filter, is used. The high capital, operating and maintenance cost of these filters constitute, on the other hand, a major drawback.
The introduction of synthetic flocculants for the improvement of the performance of settlers/thickeners (Chandler, U.S. Pat. No. 4,040,954) and the development of high efficiency thickeners, such as the ones described by Bagatto et al. (U.S. Pat. Nos. 4,830,507 and 5,080,803) and by Farmery et al. (U.S. Pat. No. 5,718,510) have led to a generation of equipment capable of producing, directly in a gravity thickener, very high solids content slurries or muds which exhibit paste-like behavior.
These new settlers are limited, however, in performance by the need to maintain an underflow solids concentration which is low enough to ensure evacuation of the mud or thick slurry from the equipment. This underflow solids concentration is normally continuously monitored in order to avoid situations which would result in a severe blockage and, in many instances, the necessity to empty the reservoir by external means.
The use of positive displacement pumps to transport the material away from the thickening equipment to an appropriate disposal site can also be a limitation to the maximum solids concentration which can be tolerated in use. Although such pumps have, in principle, the capacity to transport very high solids slurries from the pressure side of the pump, the mud must be sufficiently fluid to be introduced continuously into the pump on the suction side.
Khan et al. (U.S. Pat. Nos. 5,188,739 and 5,188,740) have described a process by which a sanitary sewage sludge mixed with carbonaceous material is fed at a relatively high solids concentration into a reactor. This is achieved, as Khan et al. describe it in their patents, by the action of a pump which reduces the viscosity of the sludge by its shearing effect. In the Khan et al. patents, the material is fed directly from a centrifuge to a pump and is sufficiently fluid to be evacuated from the thickening equipment. Khan et al.""s objective is to have a material fluid enough to enter into a subsequent reactor. However, the Khan patents do not address the problem of getting the slurry into the pump in the first place.
In summary, therefore, the difficulty of handling and transferring high viscosity slurries has limited the attempts to maximize the solids contents of such slurries, and has therefore limited the advantages that the formation of such slurries could offer.
An object of the invention is to make it possible to transfer high viscosity slurries from containers with reliability and consistency.
Another object of the invention is to facilitate transfer of high viscosity slurries, thus allowing thickening equipment to be used more efficiently and effectively.
Yet another object of the present invention, at least in its preferred forms, is to provide a method of, and apparatus for, moving slurries out of reservoirs or equipment such as, but not exclusively, deep thickeners and high efficiency settlers, so that such slurries may be transferred from the equipment in which they are formed or held to destinations where they may be used, treated or discarded. Such movement may be possible when the slurries are very thick, and have paste-like consistency exhibiting shear thinning visco-plastic rheological properties (non-Newtonian fluids).
According to one aspect of the invention, there is provided a method of removing a slurry having shear-thinning visco-plastic properties from a reservoir holding a body of the slurry, the slurry having a viscosity so high that direct withdrawal of a flow of the slurry from the reservoir is difficult, the method comprising: creating a submerged region of shear-thinned slurry of reduced viscosityin the body of slurry for entraining adjacent slurry of the high viscosity; and removing from the reservoir a portion of the slurry of reduced viscosity containing entrained slurry of high viscosity. The stream of shear-thinned slurry is preferably created by withdrawing slurry temporarily from the submerged region via an outlet to form a flow of withdrawn slurry, subjecting the withdrawn slurry to shear to produce a flow of shear-thinned slurry of reduced viscosity, and returning the shear-thinned slurry of reduced viscosity to the submerged region of the body via an inlet spaced from the outlet, thereby creating a flow of slurry between the inlet and the outlet. This withdrawal, shear application and return is preferably carried out continuously, at least during the period when slurry is to be transferred from the reservoir.
According to another aspect of the invention, there is provided apparatus for holding and delivering a slurry having a viscosity so high that direct withdrawal of a flow of the slurry from a reservoir holding a body of the slurry is difficult, the apparatus comprising: a reservoir for holding a body of the slurry; means for creating a submerged region of slurry of reduced viscosity in the body to entrain adjacent slurry of the high viscosity; and means for removing from the reservoir a portion of the slurry of reduced viscosity containing entrained slurry of high viscosity. The apparatus preferably comprises a first conduit having an inlet in the reservoir for withdrawing slurry from the submerged region, shear generating means communicating with the first conduit for subjecting withdrawn slurry from the first conduit to shear, a second conduit communicating with the shear generating means, having an outlet in the reservoir, for returning shear-thinned slurry of reduced viscosity from the shear generating means to the submerged region of the body, the inlet and the outlet being spaced from each other in the region, thereby creating the stream between the inlet and the outlet.
The invention may be used with any slurry having the required shear-thinning visco-plastic Theological properties. Slurries derived from all common mineral tailings (e.g. red mud from bauxite, tailings from zinc, copper, gold, iron ore and platinum extractions, and residues from tar sands, calcium tailings, etc.), so the invention is of particular application to such slurries. The invention is also particularly suited for use on a continuous, semi-batch or batch basis (but most especially a continuous basis) in combination with (e.g. is the same vessel as) slurry thickeners or concentrators of the type discussed above.
By the term xe2x80x9cshear-thinning visco-plastic rheological propertiesxe2x80x9d used herein to describe a slurry, we mean a thixotropic slurry having a viscosity that is reduced when the slurry is subjected to mechanical shear compared to the viscosity when the slurry is formed and remains undisturbed. The slurry with which the invention is used is generally of such high solids content that it has the properties of a paste that is difficult to remove from a reservoir by conventional methods. Basically, the invention relates to any slurry of the above kind that can be made to flow suitably for removal from a reservoir under suction when subjected to shear.
By the term xe2x80x9cshearxe2x80x9d we mean a force applied to the slurry that causes mixing or turbulence sufficient to reduce the apparent viscosity of a shear-thinning slurry. Shear varies in absolute terms according to various factors, including the apparent viscosity of the mixture. It is more meaningful, therefore, to use the property xe2x80x9cyield stressxe2x80x9d do define the force required to mix the slurry. Yield stress is the force required to commence mixing to the degree required to cause shear-thinning. The invention may require the application of fairly high yield stress, e.g. in the range of 50 to 1,000 Pa.
The invention is based on the discovery that by creating a flow or stream of slurry of reduced apparent viscosity within a submerged region of a slurry of high apparent viscosity held in a reservoir, the high shear slurry can be entrained within the flow of slurry of reduced viscosity, and thereby be caused to flow and to be removed from the reservoir. Slurry of high viscosity may then move downwards to replace the slurry thus removed, and so a constant slurry transfer from the reservoir may be achieved. The flow or stream of low viscosity slurry may be created by re-circulating a shear-thinning slurry (frequently referred to hereinafter by the term of art xe2x80x9cmudxe2x80x9d) through a high-shear device, such as a pump, so that the apparent viscosity of the mud is greatly reduced, and this xe2x80x9cremoldedxe2x80x9d mud can then act as a carrier for conveying unsheared (unremolded) higher viscosity mud to the high shear device or permanently out of the apparatus.
In this preferred form of the invention, the pump or other high shear device is believed to operate by breaking bonds formed between the solids particles of the mud, including the network formed by the flocculating agent and possibly the polymer itself (normally present in such muds if produced by slurry thickeners), which contributes to the lowering of the viscosity and the yield stress of the system. The breaking of the bonds may have the effect of releasing bound water, and the released water may act as a diluent or lubricant that reduces the apparent viscosity of the mud. The remolded lower viscosity mud, which is then re-circulated through a region of the reservoir, mixes with higher viscosity mud in its immediate vicinity and entrains it, e.g. by dissolving the higher viscosity mud or by physically entraining parts of it. The pressure exerted by the column of mud above the mixing region pushes material of higher viscosity down, filling the void created by the mud swept out of the reservoir by the re-circulating low viscosity mud which acts as a carrier. In this way, a continuous, downward movement of otherwise high viscosity, quasi-static bed of mud, is established. The amount of mud moving down in the reservoir is equivalent to the amount of mud which is withdrawn from the reservoir, e.g. via a branch conduit communicating with one of the mud recirculation conduits or exiting a side of the reservoir near the region containing the remolded mud.
In an especially preferred form, the invention involves providing an outlet (hereinafter referred to as a suction point) situated on the reservoir (on the tank wall or actually within the tank) in a region where the slurry is approximately at the desired consistency (usually at the bottom end of the reservoir), and providing a conduit (referred to as the suction conduit) of relatively short length (e.g. 30 meters or less) to connect this suction point to the suction port of a pump (or other high shear generating device), and providing another (similarly short) conduit (referred to as the discharge conduit) which connects the discharge port of the pump to another point on the reservoir (called the discharge point), this discharge point being located preferably at a short distance (e.g. between 0.2 and about 10 meters) from the suction point. The pump keeps in re-circulation a less viscous, remolded mud. A branch conduit is preferably provided communicating with either the suction conduit or the discharge conduit to transfer part of the remolded mud either to a transfer pump, to any other transportation device such as a belt or a screw conveyor. The branch conduit can alternatively be located on a wall of the reservoir (or within the reservoir) in the vicinity of the remolded mud Alternatively, the recirculation pump may also serve as the transfer pump.
In another preferred aspect of the present invention, to an arrangement similar to the one described above, a low shear stirrer of any form or shape can be incorporated into the reservoir to further reduce the viscosity of the mud and further break the solids bonds and flocculated network in the vicinity of the path of the re-circulating mud, i.e. in the region located between the suction point and the discharge point. Preferably, this low shear stirrer is a slow-moving mechanical device, such as, for instance, a rake or any other device to introduce additional shear and assist in moving the thick largely unsheared slurry towards and through the suction point in the reservoir. In the case of a rotational device, the term xe2x80x9clow shearxe2x80x9d preferably means a device having a rotational speed of about 0.01 to 5 rpm.
A preferred location to install the suction and discharge points is at the bottom section of a reservoir, such as a thickener or a settler. It can also be installed at the bottom section of an hydrocyclone or other equipment of similar geometry.
The power requirement of the re-circulation device is a function of the equivalent suction and discharge pipe dimension, the mass flow, the apparent viscosity of the slurry, the efficiency of the pump or other shearing device. In practice, there is a maximum distance between the suction and discharge points in the tank that should preferably be maintained for efficient operation. This maximum distance is function of the apparent viscosity of the recirculation mud between the two points. The maximum distance is dictated by efficiency of transportation of the low viscosity mud between these two points. This maximum efficient distance is normally in the order of 10 meters. On the other hand, If this distance is too short, the thick mud above may have a tendency to create a bridge over the space between the two points and block the downward movement of unremolded mud. Distances as small as 0.2 meters have been found effective. The suction and the discharge point can be placed at any angle of each other.
In the case where a low shear stirrer is provided, a convenient method of installation is to have a central support to which stirring elements are connected.
The ratio of slurry re-circulated over the net amount of mud evacuated can vary from 0.25 to 10:1 depending on the apparent viscosity of the mud after shearing. The higher the final apparent viscosity of the remolded mud, the higher this ratio may be.